There is known a distance measurement technique of calculating the depth value of a certain area of an object. In this technique, parallax images of the object from two different viewpoints are captured, and the depth value of the certain area of the object is calculated based on the captured parallax images. There is also known a technique of capturing images of an object from two viewpoints by using cameras including optical lenses whose view angle (angle of view) is wide, such as fisheye lenses. See, for example, the following documents:
Japanese Laid-open Patent Publication No. 2001-141422
International Publication Pamphlet No. WO2013/165006
When the above distance measurement techniques are applied to a mobile object such as a vehicle and all directions, from the front, back, right, and left of the mobile object, are set as the distance measurement directions, a pair of cameras needs to be arranged, for example, on each of the longitudinal and transverse sides of the mobile object. Namely, the number of cameras that need to be arranged on the mobile object is increased.